1. Field of the Invention
The present invention relates to a transmitter applied for radio communication, and a transmission control method therefor.
2. Description of the Related Art
An OFDM (orthogonal frequency division multiplexing) transmission system has taken attention as a system avoiding influence of inter-symbol interference, even in a multi-path transmission environment. FIG. 1 shows a configuration of a transmitter applied in the OFDM transmission system.
A transmitter 1 shown includes a symbol generating part 2 to which information bits are input; a S/P (serial/parallel) converting part 3 connected with the symbol generating part 2; an IFFT (inverse fast Fourier transform) part 4 connected to the S/P converting part 3; a P/S (parallel/serial) converting part 5 connected to the IFFT part 4; a GI adding part 6 connected to the P/S converting part 5 and an antenna 7 connected to the GI adding part 6.
When information bits are input to the symbol generating part 2, the symbol generating part 2 carries out, on the input information bit series, error correction coding, interleaving, symbol mapping and so forth, as in a single carrier transmission system, so as to generate transmission symbols, which are then input to the S/P converting part 3. The S/P converting part 3 converts the input transmission symbols in a serial form into a parallel form, and inputs the thus-obtained signal into the IFFT part 4. The IFFT part 4 converts the input signal into an orthogonal multi-carrier signal, and inputs the thus-obtained signal to the P/S converting part 5. The P/S converting part 5 converts the thus-input signal in the parallel form into a serial form, and inputs the thus-obtained signal into the GI adding part 6. The GI adding part 6 inserts guard intervals in which part of the input signal is copied. The signal thus having the guard intervals inserted thereto is then transmitted via the antenna 7.
Upon carrying out the OFDM transmission, a signal having a very large amplitude in comparison to an average amplitude appears in the output signal of the IFFT part 4.
This problem is a feature of the multi-carrier modulation system. In the multi-carrier modulation system, many carrier signal components individually modulated may be combined. Then some of the combined signal become to have large amplitudes, while some of them become to have small amplitudes. The possible maximum peak power amounts to a value larger than the average power by a factor of the number of subcarriers.
The PAPR (peak to average power ratio) may in particular become problematic due to characteristics of a transmission amplifier. In the transmission amplifier, a range in which input/output characteristics of the amplifier are linear is limited. When a signal beyond the linear range is input, an output waveform is distorted accordingly. Thereby, a problem such as degradation in the transmission quality, increase in power radiation to the outside of the band, or such may occur. Further, it is known that, when the linear range is widened, the amplification efficiency degrades. It is preferable that the PAPR of transmission signal is low.
As a method of reducing the PAPR, clipping (+filtering) (for example, see the non-patent document #1, listed below); a PTS method (for example, see the non-patent document #2); and a cyclic shifting method (for example, see the non-patent document #3) have been proposed:
Non-patent document #1: X. Li and L. J. Cimini, “Effects of clipping and filtering on the performance of OFDM”, IEEE Commun. Lett., vol. 2, no. 5, pp. 131-133, May 1998;
Non-patent document #2: L. J and N. R. Sollenberger, “Peak-to-Average power ratio reduction of an OFDM signal using partial transmit sequence”, IEEE Commun. Lett., vol. 4, no. 3, pp. 86-88, March 2000;
Non-patent document #3: G. R. Hill, M. Faulkner and J. Singh, “Reducing the peak-to-average power ratio in OFDM by cyclically shifting partial transmit sequence”, Electronics Letters, vol. 36, No. 6, pp. 560-561, March 2000; and
Non-patent document #4: Miyashita, Nishimura et al., “Eigenbeam-Space Division Multiplexing (E-SDM) in a MIMO Channel”, The Institute of Electronics, Information and Communication Engineers, Technical Report of IEICE, RCS2002-53 (2002-05).
In the clipping, a peak is replaced by a signal which has a predetermined amplitude and the same phase as the original signal.
FIG. 2 illustrates a configuration of a transmitter applied when peak reduction is carried out according to the PTS method or the cyclic shifting method.
In this transmitter, a low peak IFFT part 8 is applied instead of the IFFT part 4 in the configuration of the transmitter 1 described with reference to FIG. 1.
A configuration of the low peak IFFT-part 8 is described now with reference to FIG. 3.
As shown in FIG. 3, the low peak IFFT part 8 includes a separated IFFT part 8-1, a combining part 8-2 connected to the separated IFFT part 8-1. The combining part 8-2 includes peak reducing parts 8-21 and 8-22 connected to the separated IFFT part 8-1, a peak reduction control part 8-23 connected to the peak reducing parts 8-21 and 8-22 and the separated IFFT part 8-1, and an adding part 8-24 connected to the peak reducing parts 8-21 and 8-22. The adding part 8-24 includes a plurality of adders connected to the peak reducing parts 8-21 and 8-22. An output signal of the adding part 8-24 is input to the P/S converting part 5.
In this method, the separated IFFT part 8-1 divides a plurality of input subcarriers into a plurality of groups, for example, NG groups (NG is an integer and NG>1), and carries out IFFT thereon. The configuration of the separated IFFT part 8-1 is illustrated with reference to FIG. 4 for example in which 8 points of IFFT are divided into two groups.
The separated IFFT 8-1 includes a first IFFT part 8-11 and a second IFFT part 8-12. For example, for generating temporal signals corresponding to f(0) through f(3), a signal to be transformed are input to the f(0) through f(3) of the first IFFT part 8-11, while 0 is input to f(4) through f(7). In this configuration, since the two IFFT parts 8-11 and 8-12 are used, a calculation amount becomes double compared with the case that the dividing is not carried out is required for the IFFT processing.
The IFFT output in the configuration of FIG. 1 is equal to the sum of the outputs of the first and second IFFT parts 8-11 and 8-12.
In this system applying the PAPR reduction according to the cyclic shifting method or the PTS method, the peak reducing parts 8-21 and 8-22 carry out cyclic shifting or phase rotation on the input signals [F(0) F(1) . . . F(NFFT-1)]. After that, the thus-obtained signal components are added by the adding part 8-24. The peak reduction control part 8-23 controls the cyclic shifting amount or the phase rotation amount in such a manner as to reduce the peak appearing in the output signals. Thereby, generation of a large peak is suppressed.
Further, it is noted that, when such processing is carried out in the transmitter, the shift amount of the phase rotation should be notified of to a receiver. As a method of notifying of the information, for example, a method of utilizing a control signal, a method which applies the same shift amount or phase rotation on pilot signal and data signal, or such may be applied.